Define Plead The 5th Meaning

A refusal to answer a question because the response could hold self incriminating evidence.

Not admit to being guilty or innocent, because doing either may get you into trouble.

To call in sick when actually hungover. Stems from the many people who call off on the 5th of July.

Pleading the 5th means to not self-incriminate yourself, but it has the connotation of meaning that you did it.
The reason for this connotation is because if you didn't do it, why wouldn't you just say no?
It's a way of not saying "yes you did something" but hinting that you did, but they can't really say you hinted at it.

My friend is high and can't answer with a simple yes or no.